Chemical transformation of iron alkoxide nanosheets to FeOOH nanoparticles for highly active and stable oxygen evolution electrocatalysts

Fe3O4nanoparticles anchored on carbon nanotubes as high-performance anodes for asymmetric supercapacitors

Fe3O4/CNT composites are synthesized with ethylene glycol as solvent by a one-step solvothermal method and used as anode materials for asymmetric supercapacitors (ASC). An appropriate amount of water in ethylene glycol can accelerate the formation of Fe3O4and reduce the average size of Fe3O4to around 20 nm. However, spherical Fe3O4particles larger than 100 nm will form in pure ethylene glycol for long reaction time. The Fe3O4/CNT composite with small Fe3O4nanoparticles exhibits a high specific surface area, promoted electron transfer ability, as well as a high utilization rate of active materials. The optimized electrode shows a high specific capacity of 689 C g-1at 1 A g-1, and remains 443 C g-1at 10 A g-1. The inferior long-term cycling stability is due to the phase transition of Fe3O4and a reductive effect to form metallic Fe. An ASC using Fe3O4/CNT and NiCoO2/C composites as anode and cathode, respectively, delivers a high energy density of 58.1 Wh kg-1at a power density of 1007 W kg-1in a voltage window of 1.67 V and has a capacity retention of 63% after 5000 cycles. The self-discharge behavior of the ASC is also investigated.

New insights into pertinent Fe-complexes for the synthesis of iron via the instant polyol process

Chemically synthesized iron is in demand for biomedical applications due to its large saturation magnetization compared to iron oxides. The polyol process, suitable for obtaining Co and Ni particles and their alloys, is laborious in synthesizing Fe. The reaction yields iron oxides, and the reaction pathway remains unexplored. This study shows that a vicinal polyol, such as 1,2-propanediol, is suitable for obtaining Fe rather than 1,3-propanediol owing to the formation of a reducible Fe intermediate complex. X-ray absorption spectroscopy analysis reveals the ferric octahedral geometry and tetrahedral geometry in the ferrous state of the reaction intermediates in 1,2-propanediol and 1,3-propanediol, respectively. The final product obtained using a vicinal polyol is Fe with a γ-Fe2O3 shell, while the terminal polyol is favourable for Fe3O4. The distinct Fe-Fe and Fe-O bond lengths suggest the presence of a carboxylate group and a terminal alkoxide ligand in the intermediate of 1,2-propanediol. A large Fe-Fe bond distance suggests diiron complexes with bidentate carboxylate bridges. Prominent high-spin and low-spin states indicate the possibility of transition, which favors the reduction of iron ions in the reaction using 1,2-propanediol.

γ-FeO(OH) with Multi-surface Terminations Intrinsically Active for Electrocatalytic Oxygen Evolution Reaction

Due to poor conductivity, electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas in-situ derived FeO(OH) from the iron based electro(pre)catalyst shows superior oxygen evolution reaction (OER). Use...

High-Efficiency Anion-Exchange Membrane Water Electrolyzer Enabled by Ternary Layered Double Hydroxide Anode

Developing high-efficiency and low-cost oxygen-evolving electrodes in anion exchange membrane (AEM) water electrolysis technology is one of the major challenges. Herein, it is demonstrated that the surface corrosion of a conventional Ni foam electrode in the presence of Fe³⁺ and V³⁺ cations can transform it into an electrode with a high catalytic performance for oxygen evolution reaction (OER). The corroded electrode consists of a ternary NiFeV layered double hydroxide (LDH) nanosheet array supported on the Ni foam surface. This NiFeV LDH electrode achieves an OER current density of 100 mA cm^-2 at an overpotential of 272 mV in 1 m KOH, outperforming the IrO₂ catalyst by 180 mV. Density functional theory calculations reveal that the unique structure and the presence of vanadium in NiFeV LDH play a key role in achieving improved OER activity. When coupled with a commercial Pt/C cathode catalyst, the resulting AEM water electrolyzer achieves a cell current density as high as 2.1 A cm^-2 at a voltage of only 1.8 V_cell in 1 m KOH, which is similar to the performance of the proton exchange membrane water electrolyzer obtained from the IrO₂ and Pt/C catalysts pair.

Effects of Annealing Temperature on the Oxygen Evolution Reaction Activity of Copper-Cobalt Oxide Nanosheets

Developing high performance, highly stable, and low-cost electrodes for the oxygen evolution reaction (OER) is challenging in water electrolysis technology. However, Ir- and Ru-based OER catalysts with high OER efficiency are difficult to commercialize as precious metal-based catalysts. Therefore, the study of OER catalysts, which are replaced by non-precious metals and have high activity and stability, are necessary. In this study, a copper–cobalt oxide nanosheet (CCO) electrode was synthesized by the electrodeposition of copper–cobalt hydroxide (CCOH) on Ni foam followed by annealing. The CCOH was annealed at various temperatures, and the structure changed to that of CCO at temperatures above 250 °C. In addition, it was observed that the nanosheets agglomerated when annealed at 300 °C. The CCO electrode annealed at 250 °C had a high surface area and efficient electron conduction pathways as a result of the direct growth on the Ni foam. Thus, the prepared CCO electrode exhibited enhanced OER activity (1.6 V at 261 mA/cm²) compared to those of CCOH (1.6 V at 144 mA/cm²), Co₃O₄ (1.6 V at 39 mA/cm²), and commercial IrO₂ (1.6 V at 14 mA/cm²) electrodes. The optimized catalyst also showed high activity and stability under high pH conditions, demonstrating its potential as a low cost, highly efficient OER electrode material.

Hollow Fe3O4/carbon with surface mesopores derived from MOFs for enhanced lithium storage performance

Hollow metal-organic frameworks (MOFs) and their derivatives have attracted more and more attention due to their high specific surface area and perfect morphological structure, which determine their large potential application in energy storage and catalysis fields. However, few researchers have carried out further modification on the outer shell of hollow MOFs, such as the perforation modification, which will endow hollow nanomaterials derived from MOFs with multifunctionality. In this paper, hollow MOFs of MIL-53(Fe) with perforated outer surface are successfully synthesized by using SiO₂ nanospheres as the template via a self-assembly process induced by the coordination polymerization. The tightly packed mesopore structure makes the carbon outer shell of MOFs thinner, thus realizing the in-situ transformation from MOFs to hollow Fe₃O₄/carbon, which exhibits perfect capacity approaching 1270 mA h g^-1 even after 200 cycles at 0.1 A g^-1, as an anode material in lithium ion batteries (LIBs) application. This research provides a new strategy for the design and preparation of MOFs and their derivatives with multifunctionality for the energy applications.

Electrochemical Synthesis of Cation Vacancy-Enriched Ultrathin Bimetallic Oxyhydroxide Nanoplatelets for Enhanced Water Oxidation

Metal cation vacancies, a kind of structural defect, are viewed as a promising strategy for regulating the electronic properties to enhance the catalytic activity. However, the effective introduction of cation vacancies into electrocatalysts still remains a challenge. Herein, we present and elucidate a facile "fast reduction and in situ phase transformation" strategy at room temperature to simultaneously introduce abundant metal cation vacancies (cobalt vacancies and iron vacancies) into Co_0.5Fe_0.5OOH electrocatalysts. The incorporation of the Fe element could tailor the micrometer-sized ultrathin CoOOH platelets into nanometer-sized ultrathin Co_0.5Fe_0.5OOH platelets, and the tailoring process is accompanied with the generation of numerous cation vacancies. The defect degree of CoOOH could be effectively tuned by the incorporation of Fe, resulting in more active sites and lower energy barrier, and thereby the intrinsic catalytic activity of electrocatalysts was further enhanced. Compared to CoOOH, the optimized nanometer-sized ultrathin Co_0.5Fe_0.5OOH platelets (Co_0.5Fe_0.5OOH-NSUPs) require a smaller overpotential of 220 mV at a current density of 20 mA cm^-2, lower Tafel slope of 38.2 mV dec^-1, and better long-term durability without obvious decay for more than 200 h at a high current density of 40 mA cm^-2. The electrochemical performances are equal to or better than that of the reported first-class electrocatalysts. More importantly, this work provides new perspective for designing and fabricating efficient multimetal electrocatalysts in large scale.

"Transformed" Fe3S4 tetragonal nanosheets: a high-efficiency and body-clearable agent for magnetic resonance imaging guided photothermal and chemodynamic synergistic therapy

To retain the agents in tumors for cancer diagnosis and therapy, and then to remove them from the body, are key for the clinical applications of ideal inorganic theranostic agents. To meet these needs, we have developed a transformed theranostic platform, employing PVP coated Fe3S4 tetragonal nanosheets (TNSs), which could effectively accumulate in the tumor under magnetic targeting, whilst gradually transforming to small particles (∼5 nm) over three weeks. These were then effectively excreted from the body in normal physiological conditions after exerting their therapeutic effect. The aqueous dispersion of PVP coated Fe3S4 TNSs had an intense near-infrared absorption, excellent photothermal conversion efficiency (64.3%) and great T2 weighted magnetic resonance imaging properties (71.3 mM-1 S-1). In addition, Fe3S4 TNSs could realize a synergistic photothermal therapy (PTT)/chemodynamic therapy (CDT), because the localized heat produced by PTT from the defect-rich structure could enhance the Fenton process by utilizing the overproduced H2O2 in the tumor microenvironment, and in return, the produced ˙OH could inhibit tumor growth and recurrence after PPT. We thus developed a high-efficiency inorganic theranostic platform which was effectively cleared from the body. This will open up a new avenue for the design of inorganic agents for clinical applications in the future.